Vapor cell structure having cavities connected by channels for micro-fabricated atomic clocks, magnetometers, and other devices

ABSTRACT

A first apparatus includes a vapor cell having first and second cavities fluidly connected by multiple channels. The first cavity is configured to receive a material able to dissociate into one or more gases that are contained within the vapor cell. The second cavity is configured to receive the one or more gases. The vapor cell is configured to allow radiation to pass through the second cavity. A second apparatus includes a vapor cell having a first wafer with first and second cavities and a second wafer with one or more channels fluidly connecting the cavities. The first cavity is configured to receive a material able to dissociate into one or more gases that are contained within the vapor cell. The second cavity is configured to receive the one or more gases. The vapor cell is configured to allow radiation to pass through the second cavity.

TECHNICAL FIELD

This disclosure is generally directed to gas cells. More specifically,this disclosure is directed to a vapor cell structure having cavitiesconnected by channels for micro-fabricated atomic clocks, magnetometers,and other devices.

BACKGROUND

Various types of devices operate using radioactive gas or other gaswithin a gas cell. For example, micro-fabricated atomic clocks (MFACs)and micro-fabricated atomic magnetometers (MFAMs) often include a cavitycontaining a metal vapor and a buffer gas. In some devices, the metalvapor and the buffer gas are created by dissociating cesium azide (CsN₃)into cesium vapor and nitrogen gas (N₂).

SUMMARY

This disclosure provides a vapor cell structure having cavitiesconnected by channels for micro-fabricated atomic clocks, magnetometers,and other devices.

In a first example, an apparatus includes a vapor cell having first andsecond cavities fluidly connected by multiple channels. The first cavityis configured to receive a material able to dissociate into one or moregases that are contained within the vapor cell. The second cavity isconfigured to receive the one or more gases. The vapor cell isconfigured to allow radiation to pass through the second cavity.

In a second example, a system includes a vapor cell and an illuminationsource. The vapor cell includes first and second cavities fluidlyconnected by multiple channels. The first cavity is configured toreceive a material able to dissociate into one or more gases that arecontained within the vapor cell. The second cavity is configured toreceive the one or more gases. The illumination source is configured todirect radiation through the second cavity.

In a third example, an apparatus includes a vapor cell having a firstwafer with first and second cavities and a second wafer with one or morechannels fluidly connecting the cavities. The first cavity is configuredto receive a material able to dissociate into one or more gases that arecontained within the vapor cell. The second cavity is configured toreceive the one or more gases. The vapor cell is configured to allowradiation to pass through the second cavity.

Other technical features may be readily apparent to one skilled in theart from the following figures, descriptions, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure and its features,reference is now made to the following description, taken in conjunctionwith the accompanying drawings, in which:

FIGS. 1 through 4 illustrate an example vapor cell structure inaccordance with this disclosure;

FIGS. 5 and 6 illustrate another example vapor cell structure inaccordance with this disclosure;

FIGS. 7 and 8 illustrate example devices containing at least one vaporcell structure in accordance with this disclosure; and

FIG. 9 illustrates an example method for forming a vapor cell structurein accordance with this disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 9, discussed below, and the various examples used todescribe the principles of the present invention in this patent documentare by way of illustration only and should not be construed in any wayto limit the scope of the invention. Those skilled in the art willunderstand that the principles of the present invention may beimplemented in any suitable manner and in any type of suitably arrangeddevice or system.

FIGS. 1 through 4 illustrate an example vapor cell structure 100 inaccordance with this disclosure. The vapor cell structure 100 can beused, for example, to receive an alkali-based material (such as cesiumazide) and to allow dissociation of the alkali-based material into ametal vapor and a buffer gas (such as cesium vapor and nitrogen gas).However, this represents one example use of the vapor cell structure100. The vapor cell structure 100 described here could be used in anyother suitable manner.

As shown in FIGS. 1 through 3, the vapor cell structure 100 includes abottom wafer 102, a middle wafer 104, and a top wafer 106. The bottomwafer 102 generally represents a structure on which other components ofthe vapor cell structure 100 can be placed. The bottom wafer 102 is alsosubstantially optically transparent to radiation passing through thevapor cell structure 100 during operation of a device, such as amicro-fabricated atomic clock, magnetometer, or other device. The bottomwafer 102 can be formed from any suitable material(s) and in anysuitable manner. The bottom wafer 102 could, for instance, be formedfrom borosilicate glass, such as PYREX or BOROFLOAT glass.

The middle wafer 104 is secured to the bottom wafer 102, such as throughbonding. The middle wafer 104 includes multiple cavities 108-110 throughthe middle wafer 104. Each cavity 108-110 could serve a differentpurpose in the vapor cell structure 100. For example, the cavity 108 canreceive a material to be dissociated, such as cesium azide (CsN₃) orother alkali-based material. The cavity 108 can be referred to as a“reservoir cavity.” The cavity 110 can receive gas from the cavity 108,such as a metal vapor and a buffer gas. Laser illumination or otherillumination could pass through the cavity 110 during operation of adevice, such as a micro-fabricated atomic clock, magnetometer, or otherdevice. The cavity 110 can be referred to as an “interrogation cavity.”

Multiple channels 112 fluidly connect the cavities 108-110 in the vaporcell structure 100. Each channel 112 represents any suitable passagewaythrough which gas or other material(s) can flow. In this example, thereare three channels 112, although the vapor cell structure 100 couldinclude two or more than three channels 112. Also, the channels 112 hereare generally straight, have equal lengths, and are parallel to oneanother. However, the channels 112 could have any other suitableform(s).

The middle wafer 104 could be formed from any suitable material(s) andin any suitable manner. For example, the middle wafer 104 couldrepresent a silicon wafer, and the cavities 108-110 and the channels 112could be formed in the silicon wafer using one or more wet etches orother suitable processing techniques. As a particular example, thechannels 112 could be formed in a silicon wafer using a potassiumhydroxide (KOH) wet etch. The etch of the silicon wafer could also beperformed in a self-limiting manner, meaning the etch stops itself at oraround a desired depth. For instance, when a narrow mask opening is usedto expose the silicon wafer and the etching occurs at a suitable angle(such as about 54.74°), the etching can self-terminate before it etchescompletely through the silicon wafer.

Each cavity 108-110 and channel 112 could have any suitable size, shape,and dimensions. Also, the relative sizes of the cavities 108-110 andchannels 112 shown in FIGS. 1 through 3 are for illustration only, andeach cavity 108-110 or channel 112 could have a different size relativeto the other cavities or channels. Further, the relative depth of eachchannel 112 compared to the depth(s) of the cavities 108-110 is forillustration only, and each cavity 108-110 and channel 112 could haveany other suitable depth. In addition, while each cavity 108-110 isshown as being formed completely through the wafer 104, each cavity108-110 could be formed partially through the wafer 104.

The top wafer 106 is secured to the middle wafer 104, such as throughbonding. The top wafer 106 generally represents a structure that capsthe cavities 108-110 and channels 112 of the middle wafer 104, therebyhelping to seal material (such as gas) into the vapor cell structure100. The top wafer 106 is also substantially optically transparent toradiation passing through the vapor cell structure 100 during operationof a device, such as a micro-fabricated atomic clock, magnetometer, orother device. The top wafer 106 can be formed from any suitablematerial(s) and in any suitable manner. The top wafer 106 could, forinstance, be formed from borosilicate glass, such as PYREX or BOROFLOATglass.

As shown here, a portion 114 of the top wafer 106 could be thinner thanthe remainder of the top wafer 106. This may help to facilitate easierUV irradiation of material placed inside the reservoir cavity 108. Notethat any wafer 102-106 in the vapor cell structure 100 could have anon-uniform thickness at any desired area(s) of the wafer(s). Also notethat the portion 114 of the top wafer 106 could have any suitable size,shape, and dimensions and could be larger or smaller than the reservoircavity 108. The portion 114 of the top wafer 106 could be thinned in anysuitable manner, such as with a wet isotropic etch.

During fabrication of the vapor cell structure 100, the bottom andmiddle wafers 102-104 could be secured together, and the middle wafer104 can be etched to form the cavities 108-110 and the channels 112(either before or after the bottom and middle wafers 102-104 are securedtogether). An alkali-based material 116 (such as cesium azide) or othermaterial(s) can be deposited into the reservoir cavity 108 as shown inFIGS. 3 and 4. Any suitable deposition technique can be used to depositthe material(s) 116 into the cavity 108. The top wafer 106 can besecured to the middle wafer 104 once the material 116 is placed in thecavity 108. At this point, the cavities 108-110 and the channels 112 canbe sealed.

At least a portion of the material 116 in the cavity 108 can bedissociated. This could be accomplished by exposing the material 116 inthe cavity 108 to ultraviolet (UV) radiation. For example, analkali-based material 116 can be dissociated into a metal vapor and abuffer gas. As a particular example, cesium azide could be dissociatedinto cesium vapor and nitrogen gas (N₂). Note, however, that othermechanisms could be used to initiate the dissociation, such as thermaldissociation. The dissociation of the material 116 creates gas insidethe reservoir cavity 108, which can flow into the interrogation cavity110 through the channels 112.

In conventional devices, material is often dissociated in a singlecavity, and the resulting gas is kept in the same cavity. Radiation canbe passed through the gas in that single cavity during operation of adevice, but residue from the original material may still exist in thatsingle cavity. This residue can interfere with the optical properties ofthe cavity and lead to device failure.

In accordance with this disclosure, the material 116 can be placed inone cavity 108 and dissociated, and the resulting gas can be used in adifferent cavity 110 during device operation. As shown in FIG. 4, anillumination source 118, such as a vertical-cavity surface-emitted laser(“VCSEL”) or other laser, could direct radiation through theinterrogation cavity 110. Even if residue exists in the reservoir cavity108, it may not interfere with the optical properties in the cavity 110.

The use of multiple channels 112 also helps to ensure that vapor cantravel from the reservoir cavity 108 into the interrogation cavity 110,even if one or more channels 112 become blocked by debris or othermaterial(s). In particular embodiments, the channels 112 could representself-height-eliminating channels fabricated using a wet etch, ratherthan a more expensive and time-consuming dry etch. This can help tosimplify the manufacture of the vapor cavity structure 100. In addition,thinning the portion 114 of the top wafer 106 through which UV radiationis directed into the reservoir cavity 108 allows for enhanceddissociation of the material 116 in the cavity 108 (possibly at reducedpower levels) while maintaining the mechanical integrity of the overalldevice.

Although FIGS. 1 through 4 illustrate one example of a vapor cellstructure 100, various changes may be made to FIGS. 1 through 4. Forexample, the vapor cell structure 100 need not include two cavities andcould include three or more cavities. Also, the cavities 108-110 andchannels 112 need not be arranged linearly, and the channels 112 neednot be straight. Any arrangement of cavities connected by channels couldbe used, including non-linear and multi-level arrangements. Further, thevapor cell structure 100 could be used with any other material(s) and isnot limited to alkali-based materials or metal vapors and buffer gases.In addition, the vapor cell structure 100 can be used in any othersuitable manner and is not limited to the use shown in FIG. 4.

FIGS. 5 and 6 illustrate another example vapor cell structure 500 inaccordance with this disclosure. The vapor cell structure 500 shown hereis similar in structure to that shown in FIGS. 1 through 4. Referencenumerals 102-110 and 114-118 are used here to denote structures that maybe the same as or similar to structures described above. In thisexample, however, channels are not formed in the middle wafer 104.Rather, one or more channels 512 are formed in the top wafer 106. Thetop wafer 106 in this example may be said to represent a “capping” layersince it can be secured to the middle wafer 104 after the material 116is inserted into the cavity 108, thereby capping the structure 500.

The channels 512 (and possibly portions of the cavities 108-110) can beetched into the top wafer 106 in any suitable manner. For example, aphotoresist mask can be formed on the top wafer 106, patterned, andbaked/cured. An isotropic wet etch, such as one using a hydrofluoricacid (HF) dip, can then be performed to etch exposed portions of the topwafer 106. The composition of the wet etch bath and the etch time can beselected to reduce the thickness of the top wafer 106 as desired. Thephotoresist layer can then be removed, and the top wafer 106 can becleaned in preparation for securing to the middle wafer 104. In thisway, the top wafer 106 need not be thinned significantly or at all overthe interrogation cavity 110, helping to preserve the mechanicalstrength of the vapor cell structure 500. The channels 512 in thecapping layer can also serve other functions, such as by serving ascondensation sites in the vapor cell structure 500.

FIG. 6 illustrates various examples of the channels and cavity portionsthat can be etched into a capping layer, such as the top wafer 106. Forexample, arrangement 602 includes portions of two unequally-sizedcavities and a single channel between the cavities. Arrangement 604includes portions of two unequally-sized cavities and two channelsbetween the cavities. Arrangement 606 includes portions of twoequally-sized cavities and three channels between the cavities.Arrangement 608 includes portions of two unequally-sized cavities andfour channels between the cavities. Arrangement 610 includes portions ofthree unequally-sized cavities and five channels coupling each adjacentpair of cavities. Arrangement 612 includes portions of threeequally-sized cavities and five channels coupling each adjacent pair ofcavities. These arrangements are for illustration only, and otherarrangements of cavities and channels (whether linear or non-linear)could be used in the vapor cell structure 500.

In particular embodiments, the top wafer 106 could be formed fromborosilicate glass, and the etch of the top wafer 106 could occur usinga hydrofluoric acid (BHF) bath. A hard mask could be used to mask thetop wafer 106. Any suitable etch, hard mask, and etch depth could alsobe used.

Although FIGS. 5 and 6 illustrate another example of a vapor cellstructure 500, various changes may be made to FIGS. 5 and 6. Forexample, the vapor cell structure 500 could include any number ofcavities and any number of channels in any suitable arrangement. Also,the vapor cell structure 500 could be used with any suitable material(s)and is not limited to alkali-based materials or metal vapors and buffergases. In addition, the vapor cell structure 500 can be used in anyother suitable manner.

FIGS. 7 and 8 illustrate example devices containing at least one vaporcell structure in accordance with this disclosure. As shown in FIG. 7, adevice 700 represents a micro-fabricated atomic clock or other atomicclock. The device 700 here includes one or more illumination sources 702and a vapor cell 704. Each illumination source 702 includes any suitablestructure for generating radiation, which is directed through the vaporcell 704. Each illumination source 702 could, for example, include alaser or lamp.

The vapor cell 704 represents a vapor cell structure, such as the vaporcell structure 100 or 500 described above. The radiation from theillumination source(s) 702 passes through the interrogation cavity 110of the vapor cell 704 and interacts with the metal vapor. The radiationcan also interact with one or more photodetectors that measure theradiation passing through the interrogation cavity 110. For example,photodetectors can measure radiation from one or more lasers or lamps.

Signals from the photodetectors are provided to clock generationcircuitry 706, which uses the signals to generate a clock signal. Whenthe metal vapor is, for example, rubidium 87 or cesium 133, the signalgenerated by the clock generation circuitry 706 could represent ahighly-accurate clock. The signals from the photodetectors are alsoprovided to a controller 708, which controls operation of theillumination source(s) 702. The controller 708 helps to ensureclosed-loop stabilization of the atomic clock.

As shown in FIG. 8, a device 800 represents a micro-fabricated atomicmagnetometer or other atomic magnetometer. The device 800 here includesone or more illumination sources 802 and a vapor cell 804. Eachillumination source 802 includes any suitable structure for generatingradiation, which is directed through the vapor cell 804. Eachillumination source 802 could, for example, include a laser or lamp.

The vapor cell 804 represents a vapor cell structure, such as the vaporcell structure 100 or 500 described above. The radiation from theillumination source(s) 802 can pass through the interrogation cavity 110of the vapor cell 804 and interact with the metal vapor. The radiationcan also interact with one or more photodetectors that measure theradiation passing through the interrogation cavity 110. For example,photodetector(s) can measure radiation from one or more lasers or lamps.

Signals from the photodetector(s) are provided to a magnetic fieldcalculator 806, which uses the signals to measure a magnetic fieldpassing through the interrogation cavity 110. The magnetic fieldcalculator 806 here is capable of measuring extremely small magneticfields. The signals from the photodetector(s) can also be provided to acontroller 808, which controls operation of the illumination source(s)802.

Although FIGS. 7 and 8 illustrate examples of devices 700 and 800containing at least one vapor cell structure, various changes may bemade to FIGS. 7 and 8. For example, the devices 700 and 800 shown inFIGS. 7 and 8 have been simplified in order to illustrate example usesof the vapor cell structures 100 and 500 described above. Atomic clocksand atomic magnetometers can have various other designs of varyingcomplexity with one or multiple vapor cell structures.

FIG. 9 illustrates an example method 900 for forming a vapor cellstructure in accordance with this disclosure. As shown in FIG. 9,multiple cavities are formed in a middle wafer of a vapor cell structureat step 902. This could include, for example, forming cavities 108-110in a silicon wafer or other middle wafer 104. Any suitable techniquecould be used to form the cavities, such as a wet or dry etch.

One or more channels are formed in the middle wafer or a top wafer ofthe vapor cell structure at step 904. This could include, for example,forming one or more channels 112 in the silicon wafer or other middlewafer 104. This could also include forming one or more channels 512 inthe top wafer 106 or other capping layer. Any suitable technique couldbe used to form the channels, such as a wet etch. The formation of thecavities and channels could also overlap, such as when the same etch isused to form both the cavities 108-110 and the channels 112.

A portion of the top wafer is thinned at step 906. This could include,for example, etching a portion 114 of the top wafer 106 in an areaadjacent to the reservoir cavity 108. Any suitable etch can occur here,such as an isotropic wet etch. The formation of channels in the topwafer and the thinning of the top wafer could also overlap, such as whenthe same etch is used to form both the channels 512 and the thinnedportion 114.

The middle wafer is secured to a lower wafer at step 908. This couldinclude, for example, bonding the middle wafer 104 to the bottom wafer102. If the cavities 108-110 are formed completely through the middlewafer 104, securing the middle wafer 104 to the bottom wafer 102 canseal the lower openings of the cavities 108-110.

A material to be dissociated is deposited in at least one of thecavities at step 910. This could include, for example, depositing thematerial 116 into the reservoir cavity 108. Any suitable depositiontechnique could be used to deposit any suitable material(s) 116, such asan alkali-based material.

The top wafer is secured to the middle wafer at step 912. This couldinclude, for example, bonding the top wafer 106 to the middle wafer 104.Securing the top wafer 106 to the middle wafer 104 can seal the upperopenings of the cavities 108-110 and the channels 112, 512. At thispoint, the cavities and channels in the vapor cell structure can besealed against the outside environment.

The material is dissociated to create metal vapor and buffer gas at step914. This could include, for example, applying UV radiation to thematerial 116 through the thinned portion 114 of the top wafer 106. Thiscould also include converting at least a portion of the material 116into the metal vapor and buffer gas. Note, however, that otherdissociation techniques could also be used.

In this way, the vapor cell structure can be fabricated in a manner thatallows easier dissociation of the material 116 while maintaining thestructural integrity of the vapor cell. Moreover, the use of multiplechannels can help to ensure that gas can flow into the interrogationcavity 110, even when one or more channels are blocked.

Although FIG. 9 illustrates one example of a method 900 for forming avapor cell structure, various changes may be made to FIG. 9. Forexample, as noted above, various modifications can be made to thefabrication process. Also, while shown as a series of steps, varioussteps in FIG. 9 could overlap, occur in parallel, or occur in adifferent order.

It may be advantageous to set forth definitions of certain words andphrases used throughout this patent document. The terms “top,” “middle,”and “bottom” refer to structures in relative positions in the figuresand do not impart structural limitations on how a device is manufacturedor used. The term “secured” and its derivatives mean to be attached,either directly or indirectly via another structure. The terms “include”and “comprise,” as well as derivatives thereof, mean inclusion withoutlimitation. The term “or” is inclusive, meaning and/or. The phrase“associated with,” as well as derivatives thereof, may mean to include,be included within, interconnect with, contain, be contained within,connect to or with, couple to or with, be communicable with, cooperatewith, interleave, juxtapose, be proximate to, be bound to or with, have,have a property of, have a relationship to or with, or the like. Thephrase “at least one of,” when used with a list of items, means thatdifferent combinations of one or more of the listed items may be used,and only one item in the list may be needed. For example, “at least oneof: A, B, and C” includes any of the following combinations: A, B, C, Aand B, A and C, B and C, and A and B and C.

While this disclosure has described certain embodiments and generallyassociated methods, alterations and permutations of these embodimentsand methods will be apparent to those skilled in the art. Accordingly,the above description of example embodiments does not define orconstrain this disclosure. Other changes, substitutions, and alterationsare also possible without departing from the spirit and scope of thisdisclosure, as defined by the following claims.

What is claimed is:
 1. An apparatus comprising: a vapor cell havingfirst and second cavities fluidly connected by multiple channels; thefirst cavity configured to receive a material to dissociate into one ormore gases that are contained within the vapor cell; the second cavityconfigured to receive the one or more gases; wherein the vapor cell isconfigured to allow radiation to pass through the second cavity; asilicon wafer comprising the cavities and the channels; and an opticallytransparent wafer secured to the silicon wafer, wherein the opticallytransparent wafer is thinner vertically between the silicon wafer andthe optically transparent wafer in a location proximate to the firstcavity than at a location proximate the second cavity and wherein thefirst and second cavities do not extend into the optically transparentwafer.
 2. The apparatus of claim 1, wherein the material comprises analkali-based material to dissociate into a metal vapor and a buffer gas.3. The apparatus of claim 2, wherein the material comprises cesium azide(CsN3) and dissociates into cesium vapor and nitrogen gas (N2).
 4. Asystem comprising: a vapor cell comprising first and second cavitiesfluidly connected by multiple channels; the first cavity configured toreceive a material to dissociate into one or more gases that arecontained within the vapor cell; and the second cavity configured toreceive the one or more gases; an illumination source configured todirect radiation through the second cavity; a first wafer comprising thecavities and the channels; and a second wafer having a top surface and abottom surface where the bottom surface is secured to the first wafer,the second wafer sealing ends of the cavities and the channels whereinthe first and second cavities do not extend into the second wafer,wherein the second wafer has a recess in the top surface is in alocation proximate to the first cavity and no recess in the top surfacein a location proximate the second cavity, wherein the recess extendsonly partially through the second wafer.
 5. The system of claim 4,further comprising: clock generation circuitry configured to generate aclock signal based on the radiation directed through the second cavity.6. The system of claim 4, wherein the material comprises an alkali-basedmaterial able to dissociate into a metal vapor and a buffer gas.
 7. Thesystem of claim 6, wherein the material comprises cesium azide (CsN₃)and is able to dissociate into cesium vapor and nitrogen gas (N₂).
 8. Anapparatus, comprising: a first and a second optically transparent wafer;and a silicon wafer between the first and second optically transparentwafers, wherein the silicon wafer includes a first cavity and a secondcavity fluidly connected to each other via multiple channels to form avapor cell; the first cavity and the second cavity extend from the firstoptically transparent wafer to the second optically transparent wafer;the multiple channels extend laterally from the first cavity to thesecond cavity and vertically between the silicon wafer and the firstoptically transparent wafer; the first optically transparent wafer isthinner in a location over the first cavity than at any location overthe second cavity; and the first cavity is smaller than the secondcavity.
 9. The system of claim 8, wherein the material comprises cesiumazide (CsN₃).
 10. The system of claim 8, further comprising clockgeneration circuitry.
 11. The system of claim 8, further comprising amagnetic field calculator.